Dual fuel tank selector

[bikernut]

Can any one tell me if the above Selector Valve has power to it all the time or just for a short bit when you first turn on the key to pressurize the system and then after the engine starts the low oil pressure switch engages and powers it? I have one that seems to do that but I think the selector valve is junk as now the right tank won't work unless I disconnect the left side and jump to the right tank. I also can't find out where it gets it power to work the selector valve.
Can anyone give me a part # for the selector Valve or are they available from a parts store? any one know what they sell for? Do any of you guys have a good wiring diagram that you can copy and send me of this truck with dual tanks, I have purchased 2 books now and the wiring diagrams are junk, don't show much and I would like a more detailed look at the wiring system. This is a 1987 K2500 with a 350 V/8 and 400 tranny. thanks in advance for any help!